A known fixing device, which is configured to thermally fix a developer image on a recording sheet, includes an endless fixing belt, a nip member disposed inside the fixing belt, and a backup member, e.g., a pressure roller, disposed such that the nip member and the backup member sandwich the fixing belt therebetween. In the fixing device, a lubricant is disposed between the fixing belt and the nip member to increase sliding movement of the fixing belt on the nip member.